Undercarriages are in general held in the retracted position by a hook installed in the hold that hooks onto a roller carried by the undercarriage when the latter arrives in the retracted position. Because of deformations, assembly clearances and machining tolerances, it is necessary to provide a possibility of adjustment of the position of the roller in order to guarantee that the hook can hook onto the roller on every occasion. In general, the roller is mounted on a support that is attached to one of the elements of the undercarriage by means of a force-takeup bolt or screw that extends with clearance through an orifice in the support so that the latter can be moved laterally in order to adjust the position of the roller. Once the correct position of the roller has been found, screws immobilising the support on the undercarriage are then tightened. It is also possible to interpose shims between the support and the undercarriage element in order to adjust the height of the roller.